The invention relates to a method for the treatment, and more particularly the catalytic treatment of the exhaust gases of IC engines such as automobile engines, in which the gases resulting from combustion are passed through a preconverter associated with a main converter and designed for starting from cold as long as the engine is in a first operational condition, in which the temperature of the exhaust gases is insufficient to activate the main converter. The invention also relates to an apparatus for performing the method.
When an IC engine is started from cold large amounts of pollutants are produced. It is known practice to reduce the emission of pollutants by using a converter placed downstream from the exhaust gas outlet of the engine, the commonest form of such converter used with gasoline engines being the catalytic converter. Furthermore converters are used with diesel engines in order for instance to reduce the amount of CO, unburned hydrocarbons, and/or soot or other solid particles in the exhaust gas. The object of the invention will be more especially explained with reference to exhaust gas catalytic converters for gasoline engines, although however in effect there are similar problems to be coped with in connection with other converters for the processing of exhaust gases from any other IC engines.
Converters call for a certain minimum temperature of the reactants supplied to them in order to develop their action. In the time expiring before such minimum temperature is attained the pollutants are let off into the environment without any conversion.
Conventional three way catalytic converters contain catalytic material for the oxidation of CO and unburned hydrocarbons and also material for the reduction of NO.sub.x. Owing to the time lag in the activation of the catalytic converter before the operational temperature is reached, pollutants such as more especially CO and unburned hydrocarbons are affected, i.e. not converted, while substantial emission of NO.sub.x only takes place when conventional operating temperatures are reached.
In order to reduce this lag in the commencement of the reaction in the catalytic converter many possibilities have been considered, at least in theory, as for instance the electrical heating of the exhaust gases prior to the entry thereof into the catalytic converter, the additional heating of the catalytic converter, or keeping the catalytic converter heated up by thermal insulation.
In the case of electrical heating of the exhaust gases of the catalytic converter, the battery or generator installed regularly in automobiles is unable to supply the heavy amperage required without expensive modification.
Keeping catalytic converters hot by the thermal insulation involves the problem of overheating, more especially during operation under full load.
In order to reduce the response time of the catalytic converter there has already been a proposal to provide a small preconverter designed for the cold start condition and placed on the inlet side of the main catalytic converter adjacent to the engine so that when the exhaust gas temperature sufficient for the operation of the main catalytic converter is reached the preconverter may be disconnected by the use of bypass so that the exhaust gases pass directly to the main catalytic converter, which is further removed from the engine, in order to avoid excessive thermal effects thereon.
This arrangement involves a complex and expensive structure and there is more especially the disadvantage that in addition to the expensive catalyst-coated surface in the main catalytic converter necessary for catalytic operation there is the need for a further catalyst coated surface, i.e. in the preconverter. This has not become accepted so that at the present time when a preconverter is used the exhaust gas is at all times passed through both the preconverter and the main catalytic converter. Owing to the smaller volume this catalytic preconverter is to heat up more rapidly and thus develop its eliminating action on pollutants even before the main catalytic converter. However, it causes undesired side effects, more especially in the form of a loss in pressure with effects on the engine under full power.